Powder coating materials generally comprise a solid film-forming resin, usually with one or more colouring agents such as pigments, and optionally they also contain one or more performance additives. They are usually thermosetting, incorporating, for example, a film-forming polymer and a corresponding curing agent (which may itself be another film-forming polymer).
Application methods include electrostatic spray and fluidised-bed processes. After application, the powder is heated to melt and fuse the particles and to cure the coating. The powder coating particles which do not adhere to the substrate can be recovered for re-use so that powder coatings are economical in use of ingredients. Also, powder coating materials are generally free of added solvents and, in particular, do not use organic solvents and are accordingly non-polluting.
Powder coating materials are generally prepared by intimately mixing the ingredients, for example in an extruder, at a temperature above the softening point of the film-forming polymer(s) but below a temperature at which significant pre-reaction would occur. The extrudate is usually rolled into a flat sheet and comminuted, producing a powder with a range of particle sizes. The smaller particles, however, give rise to problems in handling and application, and such problems become more pronounced when the proportion of fine particles is high. Accordingly, manufacturers of powders coatings generally carry out classification process to reduce the “fines” fraction, although the classification processes available (utilising air classifiers) tend to remove also some larger particles, and in practice a compromise has to be reached between reduction of the fines content and avoidance of loss.
Further improvements in handling and application are brought about by addition of one or more fluidity-assisting additives. Examples include alumina or silica. WO 94/11446 describes the use of certain inorganic materials such as the combination of alumina and aluminium hydroxide as fluidity-assisting additives for powder coating materials in which at least 95% by volume of the particles are below 50 μm, and WO 00/01775 describes the use of wax-coated silica for similar and related purposes.
In general, powder coating materials may be applied by electrostatic spray processes or by fluidised-bed processes. An advantage of fluidised bed processes is that they reduce the Faraday cage effect, thereby enabling recessed portions in the substrate workpiece to be coated, and are attractive in other respects, but have the well-known disadvantage that the applied coatings are substantially thicker than those obtainable by electrostatic spray coating processes.
In traditional fluidised bed processes the substrate is preheated (typically to 200° C. to 400° C.) and dipped into a fluidised bed of the powder coating composition. The powder particles that come into contact with the preheated substrate melt and adhere to the surface of the substrate. In such processes the powder coating material acquires a charge solely by friction, i.e. it acquires a tribostatic charge. However, in so-called electrostatic fluidised bed processes, a charge is induced into the system by the direct application of electrical energy. This may be done in a number of ways. In one method, the fluidising air is ionised by means of charging electrodes arranged in the fluidising chamber or, more usually, in the plenum chamber below the porous air-distribution membrane. The ionised air charges the powder particles (corona-charging), which acquire an overall upwards motion as a result of electrostatic repulsion of identically charged particles. The effect is that a cloud of charged powder particles is formed above the surface of the fluidised bed. The substrate workpiece (earthed) is introduced into the cloud and powder particles are deposited on the substrate surface by electrostatic attraction. No preheating of the substrate workpiece is required. Such corona-charging electrostatic fluidised-bed processes are especially suitable for coating small articles, because the rate of deposition of the powder particles becomes less as the article is moved away from the surface of the charged bed. Also, as in the case of the traditional fluidised-bed process, the powder is confined to an enclosure and there is no need to provide equipment for recycling and reblending the overspray that is not deposited on the substrate.
WO 99/30838 describes an alternative (but non-corona-charging) electrostatic fluidised bed process for forming a coating on a conductive substrate, which comprises establishing a fluidised bed of a powder coating composition, immersing the substrate wholly or partly within the said fluidised bed, applying a voltage to the substrate for at least part of the period of immersion, whereby particles of the powder coating composition adhere to the substrate, withdrawing the substrate from the fluidised bed and forming the adherent particles into a continuous coating over at least part of the substrate.
Further (non-corona-charging) electrostatic fluidised bed processes are described in WO 02198577, WO 2004052557 and WO 2004052558. In all of these, the substrate is either electrically isolated or earthed. In WO 02/98577 and WO 2004052557 a voltage is applied to the conductive part of the fluidising chamber, in WO 02/98577 the substrate being conductive and in WO 2004052557 the substrate being either electrically non-conductive or poorly conductive, and in WO 2004052558 an electrically conductive electrode, to which a voltage is applied, is positioned to influence the extent to which charged particles adhere to a region of the substrate.
The processes of WO 99/30838, WO 02/98577, WO 200452557 and WO 2004052558 all rely on tribostatic (friction) charging of the powder material, the charge applied to the substrate or electrodes being used to give particles direction by setting up an electrostatic field. The term tribo-charging electrostatic processes will refer to such (non-corona) processes where a tribostatic charge is developed on the powder and voltage is applied to the system, more especially to the part to be coated or to counter-electrodes.
Corona-charging and tribo-charging electrostatic fluidised bed processes differ from other fluidised bed processes in that an electrical charge is deliberately applied to the system—in the case of corona-charging processes, to the powder, and in the case of the tribo-charging systems mentioned in the patent specifications above, to the substrate or for example to counter-electrodes within the fluidised bed that generate an electric field between the counter-electrode and the earth or oppositely-charged (or differently-charged) workpiece. In other fluidised bed processes, any electrostatic charge which arises in the system is not deliberately applied, but is as a result of friction arising incidentally in the system. This friction arising incidentally in the system however also serves as the source of charge on the powder in tribo-charging electrostatic processes.